The present invention relates to a wireless communication technology, and more particularly to a soft handoff control technology in the wireless communication technology.
In a wireless communication system, data communications via wireless terminals, such as mail sending and receiving, access to the Internet, and web browsing, are becoming more and more popular recently. A wireless communication system comprises wireless terminals that are mobile stations, wireless base stations (BS) that are fixed stations and communicate wirelessly with wireless terminals, a base station controller (BSC) that is connected to a plurality of wireless base stations for controlling them, and an external network such as a public network. The base station controller that receives data from an external network transfers the data to wireless base stations via narrowband wired communications using T1 lines (standard for digital leased line at the communication speed of 1.5 Mbps). A wireless base station that receives data transfers the data wirelessly to wireless terminals. Such data communications are carries out according to a standardized method such as the one described in “3GPP2 C.S0024-A cdma2000 High Rate Packet Data Air Interface Specification Version 3.0 Chapter 8” (p. 8-86 and P. 8-132).
A wireless terminal, which is a mobile station, is required to continue data communications while switching wireless base stations, with which the wireless terminal communicates, from one wireless base station to another as it moves around. This switching operation is called a handoff. One of handoff technologies is called a soft handoff.
A soft handoff refers to a control method that carries out a handoff with no momentary interrupt in the data communications. In a soft handoff, the wireless communication path via the target wireless base station is reserved before releasing the wireless communication path via the source wireless base station. And, after the base station controller switches the data-receiving wireless base station from one wireless base station to another, the wireless communication path via the source wireless base station is released and the communication with the target wireless base station is started.
In this case, the wireless communication path between the wireless terminal and the target wireless base station is reserved as follows. First, the wireless terminal that is moving while communicating with the source wireless base station detects that the intensity of the radio wave from the target wireless base station becomes strong enough to be measured. The wireless terminal reports the measurement result to the source wireless base station and the base station controller and, in response to this report, the base station controller reserves and acquires a traffic channel that is a wireless resource required for establishing a communication path between the target wireless base station and the wireless terminal. After successfully acquiring the traffic channel, the base station controller instructs the wireless terminal to add the communication path that uses the traffic channel.
When a wireless terminal moves toward the cell boundaries of a plurality of wireless base stations, the radio waves from a plurality of wireless base stations sometimes become strong enough to be received at the same time as the wireless terminal gets nearer to the cell boundaries. When the intensities of radio waves from a plurality of wireless base stations are measurable, the wireless terminal reports the measurement result on the a plurality of wireless base stations to the wireless base stations and the base station controller. As a result, the base station controller establishes communication paths via the a plurality of wireless base stations, one for each wireless base station on which the measurement result was reported, and creates a soft handoff state. In this specification, the soft handoff state refers to the state in which a wireless terminal has established communication paths via a plurality of wireless base stations.
According to the standard defined in 3GPP2 C.S0024-A cdma2000 High Rate Packet Data Air Interface Specification Version 3.0 Chapter 8 (p. 8-86 and P. 8-132), up to six communication paths (traffic channels) may be established at the same time in the soft handoff state.